This invention relates generally to ink jet printing mechanisms and in particular to a circuit and method for improving the reliability of ink jet printers in the presence of ink shorts.
Inkjet printing mechanisms use cartridges, often called "pens," which eject drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, ejecting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chamber is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as firing resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the firing resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image such as a picture, chart, or text.
As the ink jet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply across the print zone, with this supply being replenished through tubing that delivers ink from an "off-axis" stationary reservoir placed at a remote stationary location within the printer.
During printing, some small ink droplets may become airborne within the printer, forming what is known as "ink aerosol." Unfortunately, this ink aerosol often lands in undesirable locations on the inkjet cartridge that are not normally cleaned by the printhead service station. For example, this ink aerosol may collect along a portion of the cartridge exterior next to the electrical interconnect that sends the firing signals to the printhead. Moreover, the process of wiping the printhead often deposits ink on this portion of the cartridge adjacent to the electrical interconnect as an ink residue.
In addition to ink residue, inkjet cartridges may also suffer from ink leakage adjacent to the printhead. At the printhead, the ink supply necessarily comes in close proximity to the electrical interconnect to the die containing the firing resistors for the print head. Ink leakage from the ink supply to the adjacent electrical interconnect may penetrate to the electrical interconnect.
Beyond leaving the pen dirty with ink residue or contaminated with ink leakage, unfortunately, many inkjet inks are also electrically conductive. Any ink residue or leakage that comes in contact with the electrical interconnect has the potential for causing an electrical short circuit ("ink short") between the contacts. Ink residue deposited on the pen next to the electrical interconnect may be smeared on the contacts when the pen is removed, and then further smeared across the electrical interconnect when a new pen is installed, thus increasing the chances for a short circuit to occur.
The problem of ink shorts affecting data lines is exacerbated by the use of multiple pens that share the same data lines. Most color inkjet printers employ separate pens for black and for color. A new inkjet printer design according to the off-axis system uses four separate pens for the cyan, magenta, yellow, and black colors. Because the pens in the off-axis system are designed for a long life and thus need to be replaced only infrequently, it is possible for ink residue to build up over a long period of time. An ink short in the data line of one pen can disable the operation of remaining pens, leaving the user with the task of troubleshooting which of the four pens has the ink short. Moreover, ink shorts, particularly those between the power supply voltage and ground, can damage the printer circuitry by causing excessive current flow in the affected short circuit.
The problem of electrical short circuits in thermal ink jet printers was discussed in European Patent Application EP 0 805 028 A2 titled "Method and apparatus for detection of short circuits in thermal ink jet printers" with priority based on U.S. patent application Ser. No. 08/639385, filed Apr. 29, 1996, to Bolash et al. Bolash et al. teach energizing a data line and address line, detecting a short circuit, and inhibiting further activation of that data line. While effective for preventing damage to the printer by disabling any shorted data lines that are detected, Bolash et al. fail to address the problems of preventing damage from ink shorts between the power supply voltage and ground as well as isolating the ink short to a particular print head in thermal ink jet printers having more than one print head. Furthermore, Bolash et al. provide no teaching on adaptively re-mapping data lines or pen colors in order to improve printer reliability in the presence of ink shorts.
Therefore, it would be desirable to provide a circuit and method for improving the reliability, fault isolation, and fault tolerance of an inkjet printing mechanism in the presence of ink shorts.